This invention relates to a liquid crystal device with a ferroelectric liquid crystal adapted for unipolar driving.
In prior art, a liquid crystal device using a ferroelectric chiral smectic liquid crystal (FLC) comprises a pair of transparent substrates, a liquid crystal layer disposed between the substrates, an electrode arrangement in the form of pixels to apply electric field to the liquid crystal layer, and a polarizing plate provided on the disc. The liquid crystal layer change in its optical characteristics in accordance with the direction of electric field perpendicularly applied to pixels. With the polarizing plate and in the light of the anisotropic absorption by the liquid crystal layer whose anisotropic direction is changed in accordance with the direction of the applied electric field, the transparency of each pixel of the liquid crystal device can be controlled by applying an electric field of a suitable direction to the pixel. As a driving method for such a liquid crystal device, "AC driving method" is well-known in the art. According to this method, in advance of applying an electric field to a particular pixel to reverse the dipole moment of liquid crystal at the pixel, a weak electric field is applied in the reverse direction with a finely control of the duration and strength.
The liquid crystal layer used in such a liquid crystal device exhibits an apparent hysteresis. Namely, the optical characteristics of the liquid crystal can be kept even after removal of an applied electric field, until an opposed electric field stronger than a certain level, i.e., called a coersive electric field (Ec). Nonetheless, the liquid crystal itself does not have inherent hysteresis. The apparent hysteresis can be understood by taking into consideration the interaction between the inside surfaces of the substrates and the liquid crystal. From this mechanism, such a liquid crystal layer is called SSFLC (Surface Stabilized Ferroelectric Liquid Crystal). However, the Ec is faint so that the driving method of a liquid crystal device becomes complex, and that it is difficult to produce a grey scale.